Planar Lightwave Circuits (PLCs) are optical systems comprising one or more waveguides that are integrated on the surface of a substrate, where the waveguides are typically combined to provide complex optical functionality. These “surface waveguides” typically include a core of a first material that is surrounded by a second material having a refractive index that is lower than that of the first material. The change in refractive index at the interface between the materials enables reflection of light propagating through the core, thereby guiding the light along the length of the waveguide.
PLC-based devices and systems have made significant impact in many applications, such as optical communications systems, sensor platforms, solid-state projection systems, and the like. Surface-waveguide technology satisfies a need in these systems for small-sized, reliable optical circuit components that can provide functional control over a plurality of optical signals propagating through a system. Examples include simple devices (e.g., 1×2 and 2×2 optical switches, Mach-Zehnder interferometer-based sensors, etc.), as well as more complex, matrix-based systems having multiple waveguide elements and many input and output ports (e.g., wavelength add-drop multiplexers, cross-connects, wavelength combiners, etc.).
Common to most of these systems is a need for a PLC-based switching element. Historically, the most common switching elements are based on a device known as a thermo-optic (TO) phase shifter. A TO phase shifter takes advantage of the fact that the refractive index (i.e., the speed of light in a material) of glass is temperature-dependent (referred to as the thermo-optic effect) by including a thin-film heater that is disposed on the top of the upper cladding of a surface waveguide. Electric current passed through the heater generates heat that propagates into the cladding and core materials, changing their temperature and, thus, their refractive indices. TO phase shifters have demonstrated induced phase changes as large as 2π.
The TO phase shifter is typically included in another waveguide element, such as a Mach-Zehnder interferometer (MZI), to form an optical switching element. In an MZI switch arrangement, an input optical signal is split into two equal parts that propagate down a pair of substantially identical paths (i.e., arms) to a junction where they are then recombined into an output signal. One of the arms incorporates a TO phase shifter that controls the phase of the light in that arm. By imparting a phase difference of π between the light-signal parts in the arms, the two signals destructively interfere when recombined, thereby canceling each other out to result in a zero-power output signal. When the phase difference between the light-signal parts is 0 (or n*2π, where n is an integer), the two signals recombine constructively resulting in a full-power output signal.
Unfortunately, prior-art PLC-based switching elements have disadvantages that have, thus far, limited their adoption in many applications. First, TO phase shifters consume a great deal of power—on the order of 300-500 mW per heater element. Further, in addition to heating the core and cladding materials directly below the heater element, heat from the thin-film heater also diffuses laterally in the glass, which can lead to thermal crosstalk between adjacent waveguides. Still further, glass has a low thermal conductivity coefficient, which results in heating and cooling times that are long—typically, on the order of milliseconds. As a result, TO phase shifters are poorly suited for many applications.
To overcome some of the limitations of TO phase shifters, piezoelectric-actuated, stress-inducing (SI) phase shifters were developed. SI phase shifters integrated with conventional silica-on-silicon waveguides are disclosed, for example, by S. Donati, et al., in “Piezoelectric Actuation of Silica-on-Silicon Waveguide Devices,” published in IEEE Photonics Technologies Letters, Vol. 10, pp. 1428-1430 (1998). Such prior-art phase shifters have been shown to be able to enable optical switching on the order of a microsecond with low power dissipation.
It should be noted that a silica-on-silicon waveguide is a “low-contrast waveguide” that is characterized by only a slight difference (<1%) in the refractive indices of their core and cladding materials. Low-contrast waveguides were developed for use in telecommunications systems, where low propagation loss is critical. Low-contrast waveguides can have propagation losses less than 0.1 dB/cm. Further, the mode-field size of a low-contrast waveguide is typically well matched to that of an optical fiber, which facilitates low-loss optical coupling between them.
Unfortunately, because of this low refractive-index contrast, light is only loosely confined in the core of a low-contrast waveguide and a significant portion of its optical energy extends well out into the cladding as an evanescent tail. As a result, the mode-field profile of a light signal (i.e., the distribution of optical energy about the central axis of the waveguide) propagating in a low-contrast waveguide is quite large. In addition, low-contrast waveguides require cladding layers that are quite thick (typically, 12-25-microns thick). The need for such thick cladding layers can reduce the effectiveness of an integrated phase shifter.
In addition, the loosely confined optical energy of a light signal can leak out of the waveguide as it propagates down the waveguide—particularly at tight bends and loops. Still further, to avoid overlap of the loosely confined mode-field profiles of adjacent waveguides, low-contrast waveguides must be spaced well apart to avoid optical coupling between them.
PLCs based on low-contrast waveguides, therefore, require a great deal of chip real estate to realize any significant functionality, which results in high per chip cost. In addition, the large-bending radii requirement of low-contrast waveguides precludes realization of some waveguide components, such as large free-spectral-range ring resonators, which require small bend radii.
It would be desirable to combine SI phase shifter technology with a PLC technology requiring less chip real estate to enable low-cost, low-power, fast optical-signal control while maintaining low-loss optical coupling with external optical devices.